test(gtm): unit tests + fix unreachable gtm_only_firing status

Adds 50 unit tests covering:

- All pure-function parsers in gtm/read.py: _params_dict,
  _summarize_filter (including negate-flag rendering),
  _resolve_trigger (custom + built-in IDs), _trigger_group_member_ids,
  _element_visibility_summary (case-insensitive selectorType),
  _parse_trigger (type-specific dispatch), _parse_variable.
- audit_event_coverage status determination — one test per status
  code (ok, no_tag_no_fire, tag_paused, tag_active_but_not_firing,
  ok_auto_collected, ga4_fires_no_tag, gtm_only_firing,
  gtm_only_not_firing, auto_event_only, ga4_only).
- Insight generation for the four most common gap categories:
  missing tag, paused tag, dynamic-event tag, custom HTML tag.
- Matrix shape contracts: required keys, container summary,
  GA4 error short-circuit, alphabetical event sort.

Tests caught a real bug: the `ok` status was matched on
`in_gtm AND any_active_tag AND ga4_fires` without requiring
`in_codebase`, making `gtm_only_firing` (intended semantics:
\"event fires from a GTM tag but isn't in the codebase, possibly
stale\") unreachable. Fixed by adding `in_codebase` to the `ok`
condition. No user-facing surface area change for events that ARE
in the codebase — only events not in the codebase but with active
firing tags are affected, and they now correctly route to
gtm_only_firing instead of the over-eager ok.

234 tests pass (was 184 baseline + 50 new from this commit).

Author: illia-sapryga

src/adloop/crossref.py

@@ -634,7 +634,7 @@ def audit_event_coverage(
         ga4_fires = ga4_count > 0
         is_auto = event in _GA4_AUTO_EVENTS
 
-        if in_gtm and any_active_tag and ga4_fires:
+        if in_codebase and in_gtm and any_active_tag and ga4_fires:
             status = "ok"
         elif in_codebase and not in_gtm and ga4_fires and is_auto:
             status = "ok_auto_collected"